Hi Concerned,
Ditto a lot of the other comments above, where I could have been described exactly like your GF when I was in my 20's. Abstained through two pregnancies. In my 30's I began to avoid any activities that did not involve my ability to get my alcohol. For me, it was a full liter of cold duck every night, and at the end of the first one I rarely felt like I'd had enough. Rarely looked or acted drunk. Like her, maybe "really" drunk two or three times a year.
I, too, was very defensive if my drinking were "noticed" by someone else, although my husband shared my alcoholism with his own drinking, which was far worse than mine. That helped me justify my use as not excessive.
By my late 30's, I would refuse to go anywhere that did not involve alcohol, and by then I'd tried to quit numerous times with varying "success" -- I think the longest I ever made it was 6 weeks without drinking. By that time, I'd decided I had "proven" I could handle alcohol and the pattern of daily drinking would re-emerge.
The last glass of the evening thing doesn't surprise me. My guess is that you only see how much of her drinking she allows you to see. At the end of my drinking career (I quit over 20 years ago now), I would commonly have an extra bottle of vodka or some other alcohol around so I could drink more than my one bottle of cold duck without drawing attention I did not want. Don't laugh, but I kept a bottle of red food coloring in the cupboard to tint my alcohol red (if my extra booze were not red), so that it would appear that I was still drinking out of the same bottle of cold duck.
Just ask yourself this -- what "normal" drinker gets defensive about their drinking, when brought up in "light" conversation? Try planning a series of events where alcohol is not available or practical and see what she does.
Actually, the fact that you are worried means that her drinking has ALREADY affected her life in negative ways. That does not describe a "normal" drinker.
Good luck. She will stop ONLY when she wants to. Alcoholics will rarely stop for any other reason.